One thing you can do is constant water changes to control the fish waste and ammonia. Gold fish are very messy eaters and produce more ammonia than any other fish. I hope you have a very good working filter, a biological filter. If you do, then you will be ok, just like at the pet store that keeps a big fishes in a small tank.
Another thing to think about. All fishes give off body slime. This body slime is what limit thier natural growth. Without water changes to remove that, it will be congested to the point where fish start to keep growing but at a deformed shape. Deformed shape I am speaking of is the un natural body shape they were to develop as. That is the one little reason behind tank size, shape and water changes that makes for a healthy fish.
So it is easier to maintain water quality by getting a bigger tank.
I hope this helps.